Keeping Good Product From Reject Container

Getting excessive good product into the reject container is a common concern of many rotary sieve users. In almost all cases, the problem can be traced back to operational misuse. When a high rate of "overs" occurs, the first place to check is the screen. Is the screen the proper size? Is the screen tensioned correctly? Improper screen size will limit the yield of good product. An improperly tensioned screen will limit passage of good product through the screen. Always refer to your maintenance manual for tips on proper screen tensioning.

Another reason for excessive "overs" can be traced to product over-feed. In normal operation, about two-thirds of the screen area should be used. The additional one-third can handle small product surges. If the feed rate is too high and all of the screen is being used under normal operation, the screen cannot handle the surges, and this additional product is pushed into the "overs" container.

Although less common, an additional cause of excessive "overs" is screen blinding. Screen blinding usually occurs when the product contains small amounts of moisture. Product fed to a rotary sieve must be dry or the screener will not function properly.

Screen Blinding

The two biggest causes of screen blinding are loose screens and moisture in the product. Always check screen tension when making an operational check or when installing a new screen. Loose screens will not stay clean under normal operations, and a plugged screen is the result.

Moisture in the product will cause the product to agglomerate and stick to the screens, also resulting in a plugged screen. One other cause of screen blinding may be rotor speed. If the belt drive tension is too low, the belts will slip, causing the rotor to slow down, also resulting in a plugged screen.

Vibrating Sifter

The main cause of vibration is usually a worn or damaged bearing. A good preventive maintenance program should replace these bearings at regular intervals. This can save precious downtime and loss of production. Vibrations can also be traced to improperly assembled rotors and bearing housings. Loose bolts or nuts on component parts may also play a role in vibration. Be sure all components are properly tightened before operating any unit.

Air Pressure Affects Sieves

Yes, air pressure does have a negative effect on sieve operation. If pressurized air comes in with the feed, it will push good product into the "overs" container. If the pressurized air comes from a pneumatic take-away conveyor, the throughput of the unit may be compromised.

Protecting the Bearings

Most rotary sieves are designed with two types of seals for the bearings. The first is a lip-type seal, which is a mechanical type of seal. The other seal is a lantern ring, which is an air seal. Both of these seals must be in good working order to provide complete protection to the bearings. Typically, each of the sieve's air seals requires 3 to 5 psi of clean, compressed air to operate properly. Note that the required amount of air to each seal must be more than the amount of air pressure the sieve system is experiencing. It is also helpful to have an in-line airflow gauge to make sure there is the proper flow of air to the seals.